Properties of Nonequilibrium Materials
•Kinetically Disordered Intermetallic Compounds
Nonequilibrium materials can be produced in many ways. In some of the more interesting nonequilibrium materials that we have produced, the challenge is to examine their properties. In other cases the challenge is in the production of the material.
By rapid solidification we have produced chemically disordered versions
of the intermetallic compounds Ni3Al and
Ni2TiAl that, in equilibrium, are ordered
all the way to the melting point [1-4]. These disordered compounds are
predicted to have different properties than their ordered compounds. For
example, the thermal expansion coefficients and elastic moduli of ordered
and disordered Ni3Al are predicted to be
significantly different. Additionally, the lack of ductility of intermetallic
compounds is blamed on the long-range order, does not permit dislocations
with the smallest burgers vector to form (except as partials connected
by anti-phase boundaries which inhibit their motion). The ability to compare
the properties of microstructurally and chemically identical ordered and
disordered materials will permit Yucong
Huang and Michael Aziz
to see whether this is indeed the case. If so, kinetically disordering
a compound by rapid solidification (e.g., by spraying it to create a powder
and then compacting the powder) may permit parts to be fabricated to net
shape while still ductile. Alternatively, a laser surface treatment may
disorder and soften a surface layer, which could then be removed by a cutting
tool. Subsequent high-temperature annealing could bring back the long-range
order and, with it, the high-temperature strength and creep resistance
that makes ordered intermetallic compounds so attractive for high-temperature
Fig. 1. X-ray diffraction scan showing kinetic disordering of intermetallic compound Ni3Al, which in equilibrium is ordered all the way up to its melting point, by pulsed laser melting and rapid solidification. Superlattice peak (220) disappears after laser treatment while fundamental peak (110) remains, showing that the material has been kinetically disordered. Long-range order is recovered with a subsequent furnace anneal.
Contents1. W.J. Boettinger and M.J. Aziz, "Theory for the Trapping of Disorder and Solute in Intermetallic Phases by Rapid Solidification", Acta Metallurgica 37, 3379-3391 (1989).
2. W.J. Boettinger, L.A. Bendersky, J.A. West, M.J. Aziz and J. Cline, "Disorder Trapping in Ni2TiAl", Materials Science and Engineering A 133, 592-595 (1991).
3. J.A. West, J.T. Manos and M.J. Aziz, "Formation of Metastable Disordered Ni3Al by Pulsed Laser-Induced Rapid Solidification", Materials Research Society Symposia Proceedings 213, 859-864 (1991).
4. J.A. West and M.J. Aziz, "Kinetic Disordering of Intermetallic Compounds by Rapid Solidification", The Metallurgical Society Symposia Proceedings, Kinetics of Ordering Transformations in Metals, edited by H. Chen and V.K. Vasudevan (TMS, Warrendale, PA, 1992), pp. 177-184.
There is currently significant experimental [1, 2] and theoretical [3,
4] interest in the synthesis and properties of carbon nitride materials
due in part to the early prediction that a solid with the b
- Si3N4 structure, b
- C3N4, would have a hardness
rivaling that of diamond . Until now, the majority of experimental studies
have centered on low-pressure film growth. While these studies have led
to C-N materials with a wide range of compositions, including a well-defined
C2N phase with some diamond-like properties , the
local C bonding in all materials evaluated is predominantly sp2
that is typical of low-density, graphitic structures. The uniform tetrahedral
sp3 C bonding expected for pure b
- C3N4 or other high-density
phases has not yet been achieved in low-pressure studies. Andrew Stevens,
Charles Lieber, Carl Agee and Michael
Aziz are exploring synthesis routes using high pressure and temperature,
which was the first way that artificial man-made diamond was produced.
Sufficiently high pressure should stabilize the denser sp3-bonded
phases, but it is not known how high the pressure must be. Also, it is
not known whether the kinetic barriers to the formation of an sp3
-bonded carbon nitride phase can be overcome at temperatures below the
point that an sp2 -bonded precursor decomposes into
carbon and N2.
The pressure-temperature decomposition boundaries for one particular
precursor have recently been determined . Above a critical temperature,
decomposition proceeds extremely rapidly. Significantly, this critical
temperature is rather low (in the vicinity of 600 ¡C) over the pressure
range (0-20 GPa) examined. For these pressures, apparently the barrier
leading to N2 formation is lower than the carbon sp2
to sp3 transformation essential for conversion to
ultrahard carbon nitride. The formation of N2 is a
local event with a large thermodynamic driving force and is expected to
be essentially irreversible. Hence, we believe that kinetics are likely
to play a dominant role in the synthesis of sp3 -
bonded carbon nitrides. In theis regard it is encouraging that the critical
temperature increases with increasing pressure (i.e., the barrier to N2
formation is increasing with pressure). Hence, higher pressures and temperatures
may lead to a successful sp2 to sp3
transforation with the following caveats: (1) well-controlled temperatures,
which are accessible through resistive or furnace heating but not through
laser heating, are required to avoid precursor decomposition and (2) composition
analysis of microcrystalline phases are essential to avoid misassignment
1. see A.J. Stevens, T. Koga, C.B. Agee, M.J. Aziz, and C.M. Lieber, "Stability of Carbon Nitride Materials at High Pressure and Temperature", J. Am. Chem. Soc. 118, 10900-10901 (1996) and references therein.
2. Z. Zhang, S. Fan, J. Huang and C.M. Lieber, "Diamondlike Properties in a Single Phase Carbon Nitride Solid", Appl. Phys. Lett. 68, 2639-2641 (1996).
3. A.Y. Liu and M. Cohen, Science 245, 841 (1989).
4. D.M. Teter and R.J. Hemley, Science 271, 53 (1996).
There is currently a lot of excitement about semiconductor nanoclusters
due in part to the prediction that small-enough clusters will act as "quantum
dots" whose photoemission wavelength can be tuned by varying the cluster
size. We are using two approaches to synthesize semiconductor nanoclusters.
Ion implantation under controlled conditions results in the formation of
nanoclusters that are, in some cases, epitaxially oriented with the matrix
. Pulsed laser ablation into an inert atmosphere also results in nanoclusters
that can be collected and their photoluminescence properties probed [2,
3]. It has even been possible to fabricate electroluminescent devices out
of thin films composed of silicon nanoclusters . We are collaborating
with Drs. Murakami and Yoshida [2-4] to better characterize and understand
the cluster formation process. With both the ion implantation and laser
ablation techniques, the current challenge is to understand the nanocluster
nucleation and growth processes well enough to use them to control and
sharpen the cluster size distribution.
1. C.W. White, J.D. Budai, J.G. Zhu, S.P. Withrow, and M.J. Aziz, "Ion Beam Synthesis and Stability of GaAs Nanoclusters in Silicon", Applied Physics Letters, 68, 2389 (1996).
2. T. Makimura, Y. Kunii, N. Ono and K. Murakami, "Visible Light Emission from SiO2 Films Synthesized by Laser Ablation", Jap. J. Appl. Phys. 35, L1703-5 (1996).
3. T. Makimura, Y. Kunii, and K. Murakami, "Light Emission from Nanometer-Sized Silicon Particles Fabricated by the Laser Ablation Method", Jap. J. Appl. Phys. 35, 4780-4 (1996).
4. T. Yoshida, Y. Yamada and T. Orii, "A Novel Electroluminescent Diode with Nanocrystalline Silicon Quantum Dots", International Electron Devices Meeting, Technical Digest, San Francisco CA 8-11 Dec 1996.